Current environmental concerns have generated interest in the use of biodegradable plastics for disposable items such as, for example, trash bags, packaging materials, eating utensils, and the like. A variety of biodegradable polymers have been proposed for such uses. Typical of such polymers include, condensation polymers, such as, for example, polyesters, polyester amides, polymers formed by ring open polymerization, e.g., lactone, lactide and lactam polymerizations, polyhydroxyalkonoates, polylactic acid and naturally occurring polymers, such as, polysaccharides, e.g., cellulosic, starch, and soy derivatives.
As used herein, the term "biodegradable", as defined in ASTM D-883, is made with reference to degradable polymers in which the degradation results from the action of micro-organisms occurring naturally such as, for example, bacteria, fungi, and algae. The biodegradability may be evidenced, for example, by the production of CO.sub.2 and associated reduction in mechanical properties, such as tensile strength and percent elongation at break. Further details are known to those skilled in the art.
Although many polymers such as those described above, are highly effective in terms of their biodegradability, they often suffer from inferior mechanical performance which has hindered their commercial viability. More specifically, when converted to film by blown film extrusion, for example, biodegradable polymers often do not have good machine direction ("MD") Elmendorf Tear Strength as measured by ASTM D-1922, transverse direction ("TD") Tensile Impact as measured by ASTM D-1822, Falling Dart Impact Resistance as measured by ASTM D-1709, MD and TD Secant Modulus as measured by ASTM D-882, and Puncture Resistance as measured by Union Carbide Test Method WC-68-L. On the other hand, when biodegradable polymers are modified to enhance their mechanical properties, their biodegradability often suffers.
As used herein, the terms "condensation polymerization" and "polycondensation" mean: (i) a polymerization reaction in which two or more molecules are combined with the generation of water, alcohol or other simple substances as by-products; and (ii) polymerization of monomers, e.g., ester and amide monomers, formed by ring opening polymerization, e.g., lactones, lactides and lactams, which do not generate water, alcohol or other simple substances as by-products.
Often, condensation polymers suitable for use as biodegradable materials are semi-crystalline in form, e.g., greater than about 30%, often greater than about 50% and more often greater than about 70% crystalline. Complete crystallization of polymers is often a slow process requiring minutes, hours or days to fully accomplish. When crystallization is desired, the temperature is held above the glass transition temperature ("Tg") and below the crystalline melting point for a time sufficient to allow the molecules of the polymer to order themselves into crystal lattices. This process is also referred to in the art as "annealing". If the crystallinity of the polymer becomes too high, the molded article from the polymer may not have sufficient toughness to be viable in a typical end use like trash bags, mulch film, molded parts and the like.
Accordingly, improved condensation polymers having enhanced mechanical properties are desired which can retain their biodegradable characteristics.